The present invention relates to samples intended to be examined under the microscope, particularly samples of printed circuit boards or cards. These samples take the form of a board having coated perforations. The invention is also aimed at processes for preparing these samples.
The printed circuits, and more particularly the thickness of the metal covering of the wall of their perforations, are checked by examining, under the microscope, a section passing through the diameter of the perforation to be checked. It is essential, to that effect, that the plane of section passes exactly through the axis of the perforation. With regard to FIG. 4 of the journal "Structure 10, Struers Nouveautes Metallographiques" of April 1985, the very great precision required is demonstrated mathematically and it is proposed that it be achieved by means of a process of preparing samples which consists in piercing beforehand reference holes in the boards, in inserting therein two rods, in suspending the boards by the rods above a mould, in filling the mould with an embedding resin up to a level below the rods, in letting the resin which is used to support the board cure and put it under the microscope, in removing the embedded sample from the mould, in removing the rods and in prepolishing, that is, in abrading, then in polishing the sample as far as the perforations.
The success of this process necessitates great precision of location and diameter of the reference holes, which have no other use, and a special flask mould forming a reference level. The positioning rods must have very low tolerances so that the samples are supported properly in the mould. They are so difficult to remove after embedding that it has been necessary to design, to that effect, special equipment called a "rod extractor". However, in addition to these difficulties of implementation, the process suffers from a serious defect: the operator has no way at all of being aware that the operations have taken place correctly. It is necessary to put blind faith at one and the same time in his ability and in the equipment. He may thus discard good boards simply because the preparation of the sample is not correct or, more rarely, believe that the boards are good when they are faulty, but that the plane of the section is at fault.
The invention offsets these disadvantages by means of a sample prepared by a simpler process, which is quicker and yet more reliable than the previous process. Above all, the operator can see immediately with the naked eye that a sample has not been prepared correctly and more particularly that the section which ought to be subjected to inspection under the microscope is faulty.